In order to adhere to the emission limits which are applicable at present, a catalytic after-treatment of the exhaust gases is required. The mixture, which consists of fuel and air, is characterized by the so-called lambda air ratio, which indicates the ratio of the current air-to-fuel mixture during combustion in the cylinder. Different lambda probes are known for measuring the concentration of oxygen in the exhaust gas. For the most part, said probes can be divided into binary and linear lambda probes. In the case of λ=1, the binary lambda probe output voltage fluctuates. In the case of the linear lambda probe, deviations from λ=1 are proportionate to the output signal.
Irrespective of the type of lambda probe used, the standard configuration for a lambda probe consists of a pre-catalytic converter sensor, a catalytic converter, and possibly a post-catalytic converter sensor. The pre-catalytic converter and the post-catalytic converter sensors are also referred to as an upstream oxygen sensor or as a downstream oxygen sensor.
A disadvantage of this arrangement is the fact that the sensor and the catalytic converter have to be built in as separate units in the exhaust gas tract.